Tyrell Cobb case not a 'whodunnit': judge

The mother of Tyrell Cobb will have to wait to learn what punishment a Queensland judge will hand down after she pleaded guilty to manslaughter over the four-year-old's death.

Heidi Strbak pleaded guilty to the charge on the basis of criminal negligence after her young son died on May 24, 2009.

But the 34-year-old faced a contested sentencing in the Brisbane Supreme Court because prosecutors claim she inflicted a fatal blow or blows to the little boy, who vomited green bile before he died.

In his submissions on Friday, defence barrister Greg McGuire said it couldn't be established to the necessary legal standard his client had been responsible for those injuries.

He said it was a "tragic reality" the exact circumstances of Tyrell's death would never be known, while also claiming the circumstantial case against Strbak's then-partner Matthew Scown was stronger than that against the boy's mother.

But Justice Peter Applegarth told the court the case was not a "whodunnit" he would be required to solve.

"This case is whether the prosecution can prove enough, on the balance of probabilities ... that your client was responsible for one of the fatal injuries," the judge told Mr McGuire.

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Emotions ran high during the hearing, with Tyrell's biological father, Jason Cobb, slamming and damaging a glass door as he left the courtroom after giving his testimony.

The court was also shown confronting post-mortem images of the child's mottled body on the first day, as well as an audio recording of a frantic triple-zero call in which Scown tells the operator the child isn't breathing.

Crown prosecutor Phil McCarthy said on Thursday Strbak had demonstrated a "capacity for deception" when telling the truth did not advance her own self-interest.

He said she told "clear lies" to conceal from police she had left her Gold Coast home to obtain cannabis on the night Tyrell died.

In contrast, Mr McCarthy said Scown - who served as a key witness in the prosecution's case - had been "frank and unguarded" when giving evidence about the child's injuries.

Scown was last month sentenced to four years' jail for manslaughter but made headlines after he walked from court given he'd already spent two years and eight months in custody.

Justice Applegarth is expected to make a decision on the contested facts before the matter returns to court in mid-December.